Automatic Tuning Resonant Capacitor to Fix the Bidirectional Detuning With ZVS in Wireless Power Transfer

The power transfer efficiency of a wireless power transfer is degraded by mismatch between LC resonant frequency and TX switching frequency. One of the drawbacks of conventional resonance tuning methods is that only increment of capacitor is allowed. Even if some conventional tunings allow both increment and reduction of capacitance, they fail zero-voltage switching (ZVS) at the increment of capacitance, which practically limits their tuning range. To solve the issue, we propose an automatic tuning resonant capacitor (ATRC), which can both increase and decrease the resonant capacitor. The tuning mosfets achieve zero-voltage turn-on (ZVS) both at the capacitor-reduction mode and the capacitor-increment mode. These two (decrement/increment both are possible, and ZVS for both modes) are the advantages compared to conventional autotuning methods. Additionally, low dv/dt turn-off is achieved at capacitor-increasing mode in proposed method. Hence, the delivered power and overall efficiency is improved. The proposed method is suitable for any LC network to solve the detuning problem. A 200 W prototype is fabricated and tested experimentally. The measurement results show that the proposed ATRC can improve the overall PTE by 25%-63% point when the operating frequency is detuned ranging 75-125 kHz for 100 kHz nominal LC resonant frequency.